As large amounts of data transmission are expected from next-generation mobile communications, there is a need to widen the bandwidth to which transmission signals are arranged. However, when performing a widening of the bandwidth, the transmission signal is received on the receiving device under a large multi-path (channel) influence. That is to say, with wideband single carrier transmission, inter-symbol interference (ISI) caused by multi-paths causes performance to significantly degrade. If these multi-paths are observed in frequency-domain, this results in frequency selective fading, and as the number of paths increase, the frequency selectiveness becomes stronger.
In recent years, regarding single carrier transmission and a widely known technology as an example, the signal is converted into frequencies at the receiving device, the frequency selective fading is compensated in the frequency domain, and the frequency domain equalization suppresses inter-symbol interference by a comparatively low calculation amount after returning this to a time domain signal. However, according to frequency domain equalization, the ISI may not be completely suppressed, and so turbo equalization is attracting attention. According to turbo equalization, an ISI replica is generated from bits after the frequency domain equalization, and the ISI is removed by subtraction from the receiving signal.
Also, according to PTL 1 and PTL 2, a technology is proposed in which a portion of the frequency spectrum of the transmission data (hereafter, referred to as simply spectrum) is removed and transmitted. This is illustrated schematically in FIG. 22.
FIGS. 22(A) and (B) are schematic diagrams illustrating when a portion of the spectrum is not deleted and when it is deleted. Further, the horizontal axis in FIGS. 22(A) and (B) illustrates frequency.
In FIG. 22(A), a spectrum P11 given a reference sign P11 represents a spectrum from a single carrier transmission, and represents a spectrum transmitted without any deletion. In FIG. 22(B), a spectrum P12 given a reference sign P12 represents a spectrum from single carrier transmission, and represents a spectrum transmitted with a portion deleted (frequency components higher than fc). According to the transmission using the spectrum P12, despite being smaller, the frequency band used during the transmission is able to transmit the same amount of information as when using the spectrum P11, and so the frequency utilization efficiency may be increased.
However, according to the transmission using the spectrum P12, the spectrum is degraded (also referred to as clipping, frequency domain punctures, or deletions) when received by the receiving device. As a result, transmission performance is degraded as compared to when the entire spectrum is transmitted. Thus, according to such a transmission, the frequency transmitted with a removed spectrum has zero channel gain, that is to say, received via a channel in which the gain has been degraded by frequency selective fading, and so the performing of turbo equalization or similar has been proposed.
According to PTL 1 and PTL 2, performance restoration is attempted by a iterative equalization process, but with actual mobile communication systems, there are cases of errors occurring in transmission even after performing turbo equalization processing when there is extremely strong frequency selective fading. Thus, with many wireless communication systems, a technology called HARQ (Hybrid Automatic Repeat reQuest or Hybrid ARQ) is adopted. Receiving devices using HARQ determine whether or not data (packets) is correctly decoded by using an error detection signal added to the data by the transmitting device. The receiving device notifies the transmitting device with an ACK (acknowledge) signal when the packet has been decoded correctly, and the transmitting device transmits the next packet. Conversely, the receiving device notifies the transmitting device with a NAK (negative acknowledge) when the packet has not been correctly decoded, and the transmitting device retransmits the packet.
A retransmission method called chase combining, or CC retransmission, is a well-known example of a retransmission method.
FIG. 23 is a schematic diagram describing the CC retransmission. Further, the horizontal axis in FIG. 23 represents frequency.
In FIG. 23, a spectrum P21 given a reference sign P21 represents a spectrum of a first transmission opportunity. A spectrum P22 given a reference sign P22 represents a spectrum of a retransmission opportunity. As illustrated in FIG. 23, the CC retransmission retransmits the same data during the retransmission opportunity as that of the first transmission opportunity. According to communication systems adopting the CC retransmission, the receiving device may correctly decode the transmitted signal when noise at the receiving device or the channel state for the retransmission opportunity is different than that of the first transmission opportunity.
Also, a retransmission method known as incremental redundancy (IR), or IR retransmission, is a well-known example of a retransmission method different from the CC retransmission. The IR retransmission is a retransmission method in which punctured parity bits not transmitted during the first transmission opportunity are transmitted with priority over bits transmitted not being punctured. As different bits are transmitted according to communication systems adopting IR retransmission, the receiving device may increase the coding gain over that of the CC retransmission, which transmits the same parity bits.